latching relay with kill switch

Question

I'm working on a little circuit to place on a vehicle, the expected functionality is to add a kill switch that when pushed it will stop voltage to my load, in the schematic i represent it with a bulb. For it to work again the vehicle should have to ignition off and on again. the following schematic works ok with the following exception. if I do not press the kill switch my load will also stop having voltage once my ignition is off(this is only desired when the switch is pressed, the ignition on is the event where it resets back to on).

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

I don't think you can do quite what you specified with only one relay.

If you are prepared to modify your requirements so that the circuit is reset when the ignition is turned off (rather than off and back on again) then the solution is simple.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The modified circuit with all switching in positive lines.

^{simulate this circuit}

Figure 2. By moving the load positive to bypass the ignition switch the OP's circuit timing and operation will match that of Figure 1.

^{simulate this circuit}

Figure 3. A capacitive "kick" circuit and latch.

How it works:

• When the ignition is turned on both sides of C1 rise to +12 V. C1 is sized to provide enough energy to energise the relay.
• The relay contact will then maintain power to the coil and switch on the load.
• If KILL is pressed the relay will be de-energised. Note that since C1 has been charged via the latch circuit that KILL will have to be held for long enough for the relay to drop out. The delay is shown in the timing diagram. This 'feature' may be enough to make this an unsuitable solution.
• D1 prevents the coil energising on a negative going pulse when the ignition is turned off.

For a coil resistance of R a rough idea of the time delay is give by \$ \tau = RC \$. This will need to be at least as long as the pick-time response of the relay.

Answer 2

Well, there should be two sets of contacts, one for the real load and the other for the latching control loop. The latch circuit loop should have the normally closed "kill switch" inline so it can open the circuit. Also, there should be a pulse relay to establish the loop. So you should have something like this:

^{simulate this circuit – Schematic created using CircuitLab}

Also: some ignition switches have a set of switch contacts to ground. So the ignition pulse relay might not be needed because the ignition switch could temporary give the latching relay a ground in that situation.